1. Field of the Invention
The present invention relates generally to Radio Resource Management (RRM) of a cellular communication system having Relay Stations (RSs), i.e. a Radio over Fiber (RoF) cellular communication system, and in particular, to a method and system for variably assigning a sub-channel to a Mobile Station (MS) according to a channel state per sub-channel and the number of available channels per RS.
2. Description of the Related Art
One of the important technologies for the continuous development of a wireless communication system is related to efficient operation and distribution of frequency resources. The studies for the efficient operation and distribution of frequency resources have been briskly performed from a conventional single-hop transmission scheme, in which only a direct transmission from a Base Station (BS) per cell to a Mobile Station (MS) is allowed, to an expanded multi-hop transmission scheme. In a system supporting the multi-hop transmission scheme, a signal from a BS can be transmitted to a MS via a Relay Station (RS) or directly transmitted to the MS.
Examples of cellular systems, which are being implemented or have been realized, are a single-hop system, a repeater system supporting a repeater, and a multi-hop system. The single-hop system has a structure in which a BS exists in each cell without a repeater, and a terminal or MS is directly connected to the BS without any relay. The repeater system is to improve reception signal performance of a MS in a cell boundary area or shadow area by additionally installing a repeater between a BS and the MS. In the repeater system, a single cell is formed with a BS and a plurality of repeaters, and a MS transmits and receives a signal to and from the BS and repeater(s) at the same time. According to a link type between the BS and the repeaters, the repeaters can be divided into wired-optical repeaters and wireless-Radio Frequency (RF) repeaters. The wired repeaters have an advantage of low signal attenuation and a disadvantage of non-mobility, and the wireless repeaters have a disadvantage of amplifying and broadcasting all reception signals without distinguishing a signal desired by a MS from an interference signal. While the wireless repeaters can be installed with a cheap price, antenna isolation is needed in a network construction.
A difference between a wireless RS system and a wireless repeater system is as follows. While a wireless repeater transmits a signal by amplifying a signal from a BS and an interference signal from another cell, a wireless RS can amplify only a signal desired by a MS and perform the scheduling/Radio Resource Management (RRM) of MSs in a sub-cell formed using the RS. In other words, using a wireless RS, transmission can be performed even for MSs in a shadow area in which a BS cannot directly transmit data, resulting in an expansion of cell coverage and an increase of cell throughput.
As described above, transmission performance can be improved in a wireless RS multi-hop system compared to the single-hop or repeater system. However, the wireless RS multi-hop system requires additional transmission for a data relay compared to the single-hop system. Thus, limited resources must be shared by a plurality of sub-cells and relay links, and this may cause degradation of Quality of Service (QoS). That is, although a reception Signal to Interference pulse Noise Ratio (SINR) of a MS in a cell boundary area can be improved, since a portion of frame resources is used for relay transmission, available channel resources which can be assigned to MSs may be decreased, resulting in a decrease of throughput. One of reasons of this limitation is that data transmission/reception between a BS and a RS must be accomplished through a wireless link, and the same resource must be transmitted from the BS to the RS several times. Thus, studies of Radio over Fiber (RoF) RS technology (hereinafter, wired RS system) in which each cell is formed by connecting a BS and a RS using optical fiber having a good frequency characteristic and low data loss has been performed. A basic network configuration of the wired RS system is the same as that of the wireless RS multi-hop system except that a link between a BS and a RS or between RSs is wired.
The wired RS system has drawbacks in that expenses for optical cable installation between a BS and each RS are required, and it is difficult to move an installed RS. However, the wired RS system does not have signal loss between the BS and each RS and has a radio resource increase effect compared to the wireless RS system. In addition, by transmitting various control signals via a BS-RS wired section, a high-complex and high-efficient scheduling scheme, which is limited in the wireless RS system, can be applied. Due to these advantages, the wired RS system can achieve an increase in transmission throughput for each user and the entire cell. Since each RS acts as a small BS, the transmission throughput is increased proportional to the number of RSs in an ideal case compared to a single-hop system. Further, according to a change toward a gradual decrease of hardware costs and a gradual increase of frequency resource costs, it is predicted that the wired RS system eventually has more advantages than the wireless RS system.